1. Field of the Invention
This invention generally relates to a fabrication method of a semiconductor luminescent device, and in particular, relates to a fabrication method of a semiconductor luminescent device having a low-refractive-index region around an end face thereof from which a light is emitted.
2. Description of the Related Art
A semiconductor luminescent device such as a laser diode (LD) emitting a light is used in a field of an optical communication and an optical storage media device. In particular, there is a demand for reducing an aspect ratio of an emission angle in order to improve optical use efficiency, if an optical storage device such as CD or DVD is used for an optical pickup.
FIG. 1 illustrates an emission angle. A light emitted from the semiconductor laser 60 is enlarged in a horizontal direction and a vertical direction of a substrate constituting the semiconductor laser. In the output light, an optical intensity is highest in the center thereof and is lower away from the center in the horizontal and the vertical directions. Angles, where the optical intensities of the output light in the horizontal direction and in the vertical direction are more than half of that in the center, are referred to a horizontal emission angle θH and a vertical emission angle θV respectively. Reducing the aspect ratio of the emission angle is controlling a ratio of the horizontal emission angle θH and the vertical emission angle θV to be closer to 1.
Japanese Patent Application Publication No. 2005-19679 discloses a method of controlling an equivalent refractive index of a ridge and the both sides thereof in a window area of a ridge-structure semiconductor laser to be higher than that of a ridge and the both sides thereof in a gain area except for the window area, in order to reduce the aspect ratio of the emission angle.
Generally, the vertical emission angle θV is higher than the horizontal emission angle θH. And reducing the vertical emission angle θV is advantageous for reducing the aspect ratio of the emission angle. However, a refractive index of a clad layer is increased or a thickness of an active layer is reduced in order to reduce the vertical emission angle θV. It is preferable that confinement of a light around the active layer in the vertical direction is reduced by changing the structure of an epitaxial layer. These processes, however, cause a reduction of a gain of the active layer. Accordingly, a threshold carrier density is increased. And a threshold current is increased and an optical output power is limited by thermal saturation.
In addition, in a method of changing the vertical emission angle θV according to the structure of the epitaxial layer, it is necessary to change the structure of the epitaxial layer according to every required emission angle. Therefore it takes time to satisfy the different demands of the emission angle. Further, when the thickness of the epitaxial layer differs in the wafer plane, the emission angle changes in the wafer plane. And a defective fraction of the semiconductor laser increases.